Although work on improved fuel economy has expanded in recent years for all classes of vehicles with the increasing cost of fuel and energy in general, fuel consumed by trucks has been growing at a faster rate than that for passenger cars, perhaps due to the more limited availability of such technology since many trucks already use efficient diesel engines, and the weight constraints associated with these vehicles.
Typical braking systems readily turn the energy of vehicle motion into heat by means friction which slows the vehicle. Complex systems have been designed to recover some of this energy and reintroduce it into the system so that the energy is not wasted. Methods for accomplishing this include mechanical storage systems such as flywheels, electrical storage of energy in batteries, and hydro-pneumatic storage of energy by compressing a gas using hydraulic fluid. These methods generally include a transmission for extracting energy from the system and delivering it to an energy storage device. In electric systems, a transmission transfers energy from the system to an electric generator which charges a battery bank. The transmission is designed to optimize the efficiency of the charging system and accommodate its specific power density. For example, a battery bank can only accept electrical energy at a specific rate. As a practical consideration, a vehicle driver must control the vehicle for the current traffic and road conditions which are constantly changing. Thus, the optimization of energy recovery through discrete components becomes difficult and upper limits to the amount of energy that can be recovered are quickly realized. Mechanical and hydraulic recovery systems are also plagued by their discrete nature, and optimization is difficult. Energy recovery systems require a high degree of variability in their energy recovery rate while maintaining high conversion efficiencies.
With the large mass associated with trucks, the regeneration and reuse of significant amounts of braking energy in hybrid subsystems can be high, which makes hydraulic propulsion and storage components attractive for truck applications since they are characterized by higher power density when compared with their electric counterparts. That is, as an energy storage device, a hydraulic accumulator has the ability to accept high rates and high frequencies of charging/discharging, both of which as stated are not favorable for batteries. However, the relatively low energy capacity of the hydraulic accumulator requires carefully designed control strategy, so that the fuel economy potential can be realized.
Hydraulic energy recovery methods currently in practice utilize a hydraulic pump with variable displacement to move fluid from a low pressure source into a high pressure region such as a hydro-pneumatic accumulator. For conventional piston-type hydraulic pumps and motors, variable displacement is achieved by mechanically and/or hydraulically changing the stroke of the pistons. The displacement control dictates the rate at which energy is recovered or removed from the moving vehicle. Once sufficient energy is recovered from the system, the high-pressure hydraulic fluid can be used to turn a hydraulic motor and redeliver the energy to the system so that the prime mover need not generate as much power. At specific energy recovery rates (flow rates through the pump), the pump and motor can operate efficiently. However, small variations from those specific recovery rates may affect the recovery efficiency; for example, at certain displacements the pump and motor can be efficient volumetrically and mechanically, but small deviations from those conditions may cause heat generation inside of the pump and fluid loss at the low-to-high pressure interface.